Water-cooled nuclear reactors and, in particular, pressurized-water nuclear reactors comprise assemblies consisting of a bundle of fuel rods of great length arranged parallel to each other and held in a framework consisting of guide tubes, spacers and two end fittings. The guide tubes are arranged in the longitudinal direction of the assembly and are connected to transverse spacers arranged at regular intervals along the length of the assembly.
At each of their ends, the guide tubes are also connected to either of the two end fittings constituting the stiffening and closure parts of the assembly.
The fuel rods of the assembly form a bundle in which the rods are parallel to each other and arranged, in the transverse sections of the assembly, according to a uniform lattice determined by the spacers. Certain positions in the lattice are occupied by guide tubes which are generally connected rigidly to the spacers.
The guide tubes have a length which is greater than the length of the fuel rods and are placed in the bundle so as to have a part which projects relative to the bundle of fuel rods at each of their ends. The end fittings are fastened to these projecting parts of the guide tube so as to ensure the closure of the assembly at each of its ends.
The fuel rods consist of sintered pellets of nuclear fuel material stacked inside a metal sheath isolating the pellets from the fluid surrounding the fuel assembly. In the case of a sheath rupture in a fuel assembly rod, it is necessary to replace this rod very quickly to avoid leakages of radioactive product into the reactor coolant fluid. To gain access to the fuel rods and to perform their replacement, it is necessary to dismantle one of the end fittings of the assembly, and this involves removing the connections between the corresponding ends of the guide tubes and the end fitting.
The end fittings comprise passage holes reproducing the lattice of the guide tubes in each of which a guide tube is engaged and fastened.
In order to make it possible to replace defective rods in fuel assemblies, new fuel assemblies have been designed and developed, comprising guide tubes whose connection to at least one of the end fittings is demountable.
To perform the replacement of the defective fuel rods, the assembly is placed under water in a vertical position, in a pool such as a storage pool; the assembly rests on the bottom of the pool by means of its lower end fittings. The other, upper end fitting is accessible under a certain depth of water from above the pool.
In a known type of demountable fuel assembly, the parts of the guide tubes engaged in the upper end fitting of the assembly comprise a radially expandable part which can be, for example, attached to the end of the guide tube. This expandable part can consist of a split ring having a part which projects radially outwards and which is intended to be housed in a cavity of corresponding shape machined inside the end fitting, in the passage hole for the guide tube. A locking sleeve introduced inside the guide tube produces the radial expansion of the split ring and the interlocking of the guide tube by its radially projecting part which fits inside the cavity machined in the end fitting.
The guide tube is engaged in the hole passing through the adapter plate of the end fitting only over a certain length, the remaining part of the hole, above the guide tube, opening out onto the upper face of the adapter plate of the end fitting.
There is a known demountable connection for a guide tube of a fuel assembly of the type described above, comprising a locking sleeve having a ring ensuring the expansion of the guide tube and extended axially by a fastening shell which, when the locking sleeve is fitted into the guide tube, is housed in the part of the hole situated above the guide tube and opening onto the upper face of the adapter plate.
Radial cavities are provided in this part of the hole of the adapter plate and, after the locking sleeve has been fitted into the guide tube, the fastening shell is distorted so that the distorted parts of this fastening shell enter inside the cavities to produce the axial and rotational locking of locking sleeve.
Efficient fastening of the guide tube is thus obtained by means of operations which can be performed without difficulty from above the assembly.
However, the dismantling of the guide tube makes it necessary, as a first step, to perform the extraction of the locking sleeve, which is held in the end fitting by the fastening shell. This operation can be performed by a tool which is introduced into the sleeve and which comprises radially moveable parts which can be placed under the lower end of the sleeve. Traction is applied to the tool to allow the fastening shell to be unlocked and the sleeve to be extracted from the guide tube.
This operation of extracting the locking sleeve before dismantling the end fitting of the assembly requires the use of complex tooling whose positioning, in the axial direction, inside the opening of the end fitting and of the sleeve, must be adjusted with great accuracy, so that the movable parts come to bear on the lower end of the sleeve when traction is applied to the tool. In addition, it is very difficult to check that the tool has been correctly fitted inside the sleeve, from the upper level of the pool.
Furthermore, the extraction of the sleeve is produced by a thrust on its lower end, with the result that this sleeve is liable to undergo some buckling, which makes the extraction more difficult.
Finally, the distorted regions of the fastening shell entering the cavities of the end fitting are extracted from the cavities only with difficulty when a thrust is applied to the lower end of the sleeve.
There has also been disclosed in EP-A-0.098.774 a locking sleeve comprising an annular groove machined in the inner part of the sleeve, in an upper radially widened part of the locking sleeve also comprising the deformable shell. Such a locking sleeve has some advantages, as the extraction of the sleeve may be effected by introducing a tool in the annular groove and by exerting traction on the sleeve through a part of its inner bore.
However, such a sleeve, having a radially widened upper part and a radially extending outer rim separating the upper part from the lower locking part of the sleeve, requires the end fitting of the assembly to be specially machined, at the level of the opening receiving the guide tube. Additionally, the shape of the sleeve requires more complicated and expensive forming or machining operations.